Optical switching apparatus and optical switching method

ABSTRACT

An optical switching apparatus includes an optical switch having a plurality of input ports and output ports, optical amplifiers, monitor circuits, optical amplifiers monitor circuits, and a controller that controls the optical switch. The optical amplifiers are connected to the input ports of the optical switch. The monitor circuits are connected to the output ports of the optical switch. The controller selects one of the plurality of the monitor circuits based on predetermined rules to obtains the loss at the output ports and/or the differential loss between the channels of the optical switch. The controller further selects one of the optical amplifiers based on the configuration of the optical switch to compensate the loss and the differential loss among the different channels of the optical switch by pre-amplifying the optical signals before they reach the input ports of the optical switch.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an optical communication deviceand methods of using this device. In particular, the present inventionrelates to an optical switching apparatus suitable for switching andoutputting optical signals received from a plurality of opticaltransmission lines to other optical transmission lines, and methods forusing this apparatus.

[0003] 2. Prior Art of the Invention

[0004] To handle the sudden increase in data traffic through theInternet, etc. and the quickly growing demands for multimediacommunication of images, sound and data, much progress has been made toincrease the speed and the capacity of the transmission lines andtelecommunication network nodes. To achieve a higher transmission speed,optical communication devices and optical fiber transmission lines aregenerally used to transmit signals between telecommunication networknodes.

[0005] In recent years, to handle the ever increasing speed ofcommunication networks and to improve the capacity of communicationdevices, these communication networks and devices, use optical switchingapparatuses such as optical cross-connects (hereafter, referred to asOXC) and optical add-drop multiplexing apparatuses (hereafter, referredto as OADM), which implement switching processes such as switching oftransmission lines and switching of circuits without converting opticalsignals to electric signals before processing the signals as in theconventional communication devices.

[0006] The OXC or OADM typically includes optical switches as its maincomponents. At present, since a single stage high-capacity opticalswitch is not commercially available, a high-capacity optical switch isusually implemented through a multi-stage combination of thecommercially available low-capacity optical switches such as 2×2 or 8×8switches. The optical signal power loss and differential loss among thechannels of a commercial low-capacity optical switch might reach fromseveral dB to more than ten dB. These losses between the channelsnight-be even larger for a high-capacity switch including a multi-stagecombination of the commercially available low-capacity optical switches.Typically, an optical communication system includes optical transmittersand optical receivers before and after optical switches. Since theseoptical transmitters and receivers have limited optical transmissionoutput powers, sensitivities and dynamic ranges, compensation isgenerally required for the optical switch loss and differential lossbetween the channels.

[0007] Several methods have been proposed to solve this problem. In “AFrequency Multiplexed Routing and Selecting Hybrid Switch,” Denshi JohoTsushin Gakkai [Electronic Information and CommunicationAssociation]/Tsushin Society Taikai [Communication Society Conference(1999)]/B-12-17 (Reference A), a method is disclosed to compensate forthe losses by placing optical amplifiers in the middle and/or at theoutput of the multi-stage optical switches. In “Power Control in ADMNode Using High-speed Compact-size Optical Spectrum Monitor,” DenshiJoho Tsushin Gakkai [Electronic Information and CommunicationAssociation]/ Tsushin Society Taikai [Communication Society Conference(1997)]/B-10-101 (Reference B), it is disclosed that awavelength-division-multiplexed (WDM) optical signal is firstwavelength-demultiplexed by an OADM into an optical signal with multiplewavelengths, and that after controlling the optical amplitude for eachof the wavelengths using variable optical attenuators, the signals areagain wavelength-division-multiplexed. In this method, the amplitude foreach wavelength is controlled based on the results of multiplex signalspectrum monitors after wavelength-division-multiplexing.

[0008] Kokai Patent Journal No. HEI 11 [1999]-32010 (Reference C) to theinventor of the present application discloses an OXC containing severaloptical switches and a few optical amplifiers between the opticalswitches, wherein the optical signal amplitude is controlled using aconfiguration wherein the amplification of optical signals is adjustedwith the optical amplifiers, which is in turn controlled by theamplitude of the output optical signals.

[0009] At present, a high-capacity optical switch is usually realized bycombining commercially available low-capacity optical switches inmulti-stages. Therefore, it is necessary to appropriately calibrate andinstall an optical transmission line from the output port of an opticalswitch at one stage to the input port of another optical switch at thenext stage. Thus, maintenance is often required for those transmissionlines between the stages, and the optical transmission is interruptedduring the maintenance. Further, the interruption may also occur whenthe high-capacity optical switch is under the normal operation.

[0010] A high-capacity switching apparatus, in which optical amplifiersare placed inside or after optical switches, such as the ones disclosedin References A and C, often causes sensitivity degradation of theoptical parts on the reception side due to light surges caused by theabove described interruption of light. Thus, the configuration disclosedin Reference A or C requires a surge-preventing function in theswitching controlling unit of the optical switches and/or thecontrolling unit of the optical amplifiers. Otherwise, the disclosedhigh capacity switching apparatus needs to use high performance opticalparts such as ones with a wide dynamic range. In addition, to compensatefor the optical signals which suffer the power loss in the opticalswitches, the high-capacity switching apparatus includes opticalamplifiers placed after the optical switches. Since the spontaneousemission noise of the optical amplifiers is added to the optical signalswith a lowered power, the signal-to-noise ratio of the optical signalmay decrease and cause errors in the receiver.

[0011] Furthermore, the optical signal received by the input port of anoptical switch may take various inner paths before reaching the outputport, and the optical switch in each stage is appropriately selected andconfigured. That is, because the characteristics such as the amplitudeloss or the differential loss between the channels of each of theswitches in the multi-stage combination is different, the loss betweenthe channels of the optical switches between the input port and theoutput port will significantly vary depending on the actualconfiguration of optical switches in the multi-stage combination.Therefore, to offer a high performance large-capacity optical switch, itis desirable to realize compensation for the optical switch loss and thedifferential loss between channels that have occurred in the chosenoptical path for each input/output port. The optical switchingapparatuses as disclosed in Reference A or C, however, do not offer theabove desired function.

[0012] Meanwhile, the OADM as disclosed in Reference B adopts aconfiguration wherein the spectra of wavelength-division-multiplexedoptical signals are monitored and the loss is compensated for eachdemultiplexed wavelength in the OADM. In this configuration, since thewavelength of each signal to be compensated must be different from oneanother, the wavelengths and the multiplexing methods of the opticalsignals used as optical switching apparatuses will be limited. Inaddition, it is still not compatible with either an optical switch witha flexible configuration wherein the wavelengths monitored by themonitor units correspond to the wavelengths processed by the losscompensation units in a one-to-one fashion. It is desired variousconnections should be adopted with switching. Alternatively, an opticalswitch should have a flexible configuration with no restrictions in thewavelength of the optical signals in the multiplexing methods.

SUMMARY OF THE INVENTION

[0013] It is an objective of certain embodiments of the presentinvention to provide an optical switching apparatus with improvedcompensation functions for loss and loss differential between thechannels in the optical switching apparatus, and a method of using thisapparatus.

[0014] It is another objective of certain embodiments of the presentinvention to provide an optical switching apparatus, wherein the lossand the differential loss between the channels is easily compensatedeven when a high-capacity optical switch is involved, and a method ofusing this apparatus.

[0015] It is yet another objective of certain embodiments of the presentinvention to provide a high speed and high capacity optical switchingapparatus with a simple configuration and installation procedure, or byincluding simple hardware and software (or firmware) and a method ofusing this apparatus. This apparatus has few limitations to thewavelengths of the optical signals or the multiplexing methods, andeasily and securely compensates for the loss and differential lossbetween channels of the optical signals even while the apparatus is inservice.

[0016] It is still yet another objective of certain embodiments of thepresent invention to provide an optical switching apparatus and a methodof using this apparatus without using special high function parts orcontrol technologies with a simple configuration and installationprocedure or by means of simple hardware and software (or firmware).This apparatus has a configuration in which surges would not causeoptical parts to be degraded, and compensation for changes in the lossand loss differential among the channels is implemented even while theoptical switching apparatus is in service.

[0017] Therefore, in one aspect, the present invention relates to anoptical switching apparatus. The optical switching apparatus includes anoptical switching unit with a plurality of input ports and a pluralityoutput ports, a plurality of input signal adjusting units, and aplurality of output signal monitoring units. The optical switchingapparatus further includes a controlling unit connected to the inputsignal adjusting units, the optical switching unit and the output signalmonitoring units. The controlling units select at least one of the inputsignal adjusting units and at least one of the output signal monitoringunits based on the configuration of the optical switching unit andcontrol the amplitude of the optical signals by controlling the selectedone input signal adjusting unit based on the feedback from the selectedone output signal monitoring unit.

[0018] In a preferred embodiment, the present invention relates to anoptical switching apparatus including an optical switch with a pluralityof input ports and a plurality of output ports, a plurality of opticalamplifiers, a plurality of monitor circuits and a controller thatsupervises and controls the optical switch, the optical amplifiers andthe monitor circuits. The optical amplifiers are connected to therespective input ports of the optical switch, and the monitor circuitsare connected to the respective output ports of the optical switch. Thecontroller selects one of the monitor circuits according to prescribedrules and obtains the optical power and the differential loss betweenchannels at the output port. The controller selects and controls atleast one optical amplifier to according to the setup state of theoptical switch. The selected amplifier amplifies the optical signal tobe inputted to the input port of the optical switch, and thecompensation for the loss and differential loss between channels of theoptical switch is made for each input/output port pair of the opticalswitch. In other words, a pair of an optical amplifier on the input portside of the optical switch and an monitor circuit on the output portside of the optical switch is selected to compensate for the loss andthe differential-loss between channels of the optical switch for each ofits input/output port pair.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Preferred embodiments of the present invention will now bedescribed in conjunction with the accompanying drawings, in which;

[0020]FIG. 1 is a schematic diagram illustrating an example ofconfiguration of a communication network equipped with one preferredembodiment of the optical switching apparatus according to the presentinvention;

[0021]FIG. 2 is a schematic block diagram, which illustrates an exampleof configuration of an optical switching apparatus of the presentinvention;

[0022]FIG. 3 is an operational flow chart, which illustrates an exampleof operation of the controller;

[0023]FIG. 4 is a schematic block diagram, which illustrates anotherexample of configuration of an optical switching apparatus of thepresent invention;

[0024]FIG. 5 is an operational flow chart, which illustrates anotherexample of operation of the controller;

[0025]FIG. 6 is an explanatory diagram, which illustrates an example ofimprovement of optical signals by means of an optical switchingapparatus of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] The optical switching apparatus of the present invention andmethods for using this apparatus will be described in detail using thedrawings.

[0027]FIG. 1 is a schematic diagram of a network, which illustrates anexample of configuration of a communication network wherein the opticalswitching apparatuses of the present invention are used. The opticalswitching apparatuses or the OADM'S 100-1 ˜100-9 are interconnected withoptical fibers 200-1˜200-12 to form a communication network. Specifictypes of the optical switching apparatuses include opticalcross-connects or OXC, 100-1, 100-2, which switch, multiplex and outputthe multiplexed optical signals received from each of the input opticalfibers, 200-1˜200-5 to the output optical fibers. Other switchingapparatuses include the optical add-drop multiplexing apparatuses, OADM100-3˜100-9, which separate or insert the optical signals needed for theOADMs connected to the other OADMs from the multiplexed optical signalsreceived from the optical fibers 200-5 and 200-9. The OADM'S 100-3˜100-9transmit the optical signals through the optical fibers 200-6˜200-12among the OADMs. The communication network is formed by connecting theseoptical switching apparatuses of the present invention with the opticalfibers that transmit the optical signals having an appropriatelymultiplexed level and transmission speed that are required of thecommunication network.

[0028] Furthermore, the optical switching apparatuses of the presentinvention easily build a communication network in a flexibleconfiguration that handles various transmission speeds and levels ofmultiplexed optical signals by appropriately selecting the components inthe optical switching apparatus. For instance, the optical signal has aspeed at or above STM-0 (51.84 MHz) as specified by the ITU-Trecommendation, or the optical signal is an un-modulated direct currentlight. In addition, there is no limitation to the presence or absence ofwavelength-division-multiplexers or the number ofwavelength-division-multiplexers either. For instance, to handle 16wavelength-division-multiplexers signal counts and 4 switchingdirections, an OXC with a switching scale of around 64×64 will beneeded. In this case, it would be difficult to realize a compact signalswitching apparatus with electronic circuits when the transmission speedis 2.5 G bits per second, 10 G bits per second or faster. However, theoptical switching apparatus of the present invention easily handles theabove tasks.

[0029]FIG. 2 is a schematic block diagram, which illustrates anembodiment of the structure of the optical switching apparatus accordingto the present invention. The optical switching apparatus 100 in thisembodiment includes K pieces of optical fibers 210-1˜210-K, and220-1˜220-K for respectively inputting and outputting optical signals.The optical switching apparatus 100 offers the OXC function. Afterreceiving a plurality of wavelength multiplexed optical signals (forinstance, j wavelength) from a particular one of the optical fibers210-1˜210-K, N×N optical switch 105 switches them toward thedestinations of the optical signals. The optical signals are compensatedfor the loss and the differential loss among the channels due to theparticular one of the optical fibers 220-1˜220-K to which the opticalsignals are switched by N×N optical switch 105.

[0030] Specifically, the optical signals received from the particularone of the optical fibers 210-1˜210 -K through a particular one of theoptical input circuits 101-1˜102-K that is the realized by means ofoptical amplifiers, etc. are wavelength-demultiplexed by a particularone of wavelength demultiplexers 102-1˜102-K for each wavelength. Theoptical signal of each wavelength has its wavelength converted orregenerated by a particular one of the transponders or regenerators103-1˜103-K (TDR or RGN), which is then fed to a particular one of theoptical amplifiers 104-1˜104-N of the optical switching apparatus of thepresent invention.

[0031] After passing through a particular input ports 105-I1˜105-IN ofN×N optical switches 105, the optical signal, with its amplitude beingcontrolled by a particular optical amplifiers 104-1˜104-N depending onthe particular input circuit, is switched and transferred to one of theoutput ports 105-O1˜105-ON of the optical switches 105 depending on thedestination of the optical signal. The optical signal switched by theoptical switches 105 passes through optical splitters or opticalcouplers 106-1˜106-N. The optical signal of each wavelength is thenconverted or regenerated by transponders or regenerators 103-1˜103-K(TDR or RGN) in the same manner as the optical signal being converted orregenerated before reaching the particular input ports 105 I1˜105IN ofthe optical switch 105. Optical signals with different wavelengths arethen appropriately wavelength-division-multiplexed by the wavelengthmultiplexers 108-1˜108-K, and are then outputted to the optical fibers220-1˜220-K through the output circuits 109-1˜109-K that are realized byoptical amplifiers, etc. in the same manner as the input circuits.

[0032] Monitor circuits 107-1˜107-N monitor the state of the opticalsignals such as the optical signal amplitude and the differential lossbetween the channels at each output port of the optical switch 105. Acontroller 110 includes a monitor selector 121, for selecting one of themonitor circuits 107-1˜107-N, an amplifier controller 122 that controlseach of the optical amplifiers 104-1˜104-N, which compensate opticalsignals before their reaching the input ports 105I1˜105IN according tothe state of the outputted optical signal, an optical switch driver 123,a switch control unit 124, which sets up the optical transfer paths fromthe input ports 105-I1˜105IN to the output port 105-O1˜105ON of theoptical switch 105 and a supervisory control unit 125, which supervisesand controls the optical switching apparatus 100 by interlocking withthe monitor selector 121, amplifier controller 122, and switch controlunit 124. Furthermore, the controller 110 further includes a switchmanagement unit 126 for managing and storing the switch configurationinformation needed to set up the optical switch, the information on theactually set-up paths within the switch, etc. This controller 110further communicates with the operation management unit 150 regardingthe monitoring or controlling of the optical switching apparatus 100 ofthe present invention sets up the optical switch 105, and compensatesfor the loss and differential loss among the channels of the opticalswitch by controlling the monitor circuit 107-1˜107-N and the opticalamplifier 104-1˜104-N.

[0033]FIG. 3 is an operational flow chart, which describes the operationof the controller 110 of the optical switching apparatus 100 of thepresent invention. Using FIGS. 2 and 3, the setup in the opticalswitching apparatus according to the present invention and the operationfor compensating for the loss and the differential loss between thechannels of the optical signal will be described in detail.

[0034] (1) Setup in Optical Switch

[0035] When an optical switch 105 setup or “switching” command isreceived in step S10 from the operation management unit 150, the opticalamplifiers 104-1˜104-N related to the applicable input port is put onhold in step S11, the path from the input port to the output port of theoptical switch 105 is set up or switched in step S12. The connectionset-up information is held in the switch management unit 126 orequivalence thereof. Furthermore, the purpose of putting the opticalamplifiers 104-1˜104-N on hold in step S11 is to avoid the unstableoperation of the optical amplifiers 104-1˜104-N while the monitorcircuit 107-1˜107-N are switched. The same effect is also obtained bysetting the response speed of the optical amplifiers 104-1˜104-N at aslower speed than the switching speed of the monitor circuit.

[0036] (2) Compensation for Optical Signal

[0037] When one of the monitor circuits 107-1˜107-N is selected in stepS20 according to the predetermined rules (cycles, supervisory orders,etc.), the supervisory control unit 125 searches and selects one of theoptical amplifiers 104-1˜104-N connected to the input port correspondingto the output port that corresponds to this selected monitor circuits107-1˜107-N from the pre-held connection set-up information in step S21.

[0038] The feedback from the selected one of the monitor circuits107-1˜107-N is inputted from the monitor selector 121 to the amplifiercontroller 122. The amplifier controller 122 controls the selected oneof the optical amplifiers 104-1˜104-N by assigning the received feedbackfrom the monitor circuit to the selected one of the optical amplifiers104-1˜104-N during the selection step S21 using internal switches (notillustrated) in the amplifier controller 122 thereby to compensate forthe loss and the differential loss between the channels of the opticalsignal at the optical switch output ports.

[0039] This compensation operation comprising the steps S20, S21 and S22is repeated until all the proper optical paths are set up in the opticalswitch 105. Step S23 checks if all the optical paths have been set up.

[0040] The optical switch 105 used in the optical switching apparatus100 of the present invention is preferably an N×N high-capacity switch,which may be produced by combining multi-stage commercially availablelow capacity switches such as 2×2, 8×8, 16×16 switches. For instance,the SiO2 waveguide-based optical switch is disclosed in the OFC 2000(Optical Fiber Communication Conference) TuM2-1/207 (March 2000, p. p.207); the MEMS (Micro Electro Mechanical Systems) optical switch isdisclosed in the OECC '98 (Third Opt electronics and CommunicationsConference) 15D1-8 (July 1998, p. p. 400), the inkjet bubbletechnology-based optical switch is disclosed in the OFC 2000 TuM1-1/204(March 2000, p. p. 204); and the mechanical optical switch is disclosedin the 1997 Denshi Joho Tsushin Gakkai [Electronic Information andCommunication Association/Tsushin [Communication] SocietyConference/B-10-189. The above switches are appropriately used as thelow-capacity switch to form the N×N high-capacity switch used in theoptical switching apparatus according to the present invention. Ofcourse, if a single stage high-capacity optical switch becomescommercially available in the future, such a single stage high-capacityoptical switch may be used as the optical switch 105 used in the presentinvention.

[0041] Furthermore, the optical switching apparatus 100 of the presentinvention does not require special components for the optical amplifiers104-1˜104-N, and optical splitters or optical couplers 106-1˜106-N.Commercialized standard parts are in the optical amplifiers 104-1˜104-Nand optical splitters or optical couplers 106-1˜106-N. Also, dependingon the level of loss compensation required for the optical switch 105 orthe performance of its peripheral equipments, devices such as variableoptical attenuators, that control the characteristics of opticalsignals, are optionally used in place of the optical amplifiers104-1˜104-N.

[0042] Through its optical amplifiers 104-1˜104-N, the optical switchingapparatus 100 of the present invention properly compensates the loss anddifferential loss in optical signals after they pass through differentchannels in the optical switch 105 by monitoring the output ports of theoptical switch 105, in a high capacity optical switch. In addition, thecompensation to the loss and differential loss of the optical signals ismade while the controller is in the process of selecting the monitorcircuit. Furthermore, an optical switching apparatus containing a highcapacity optical switch properly compensates for the loss and thedifferential loss between the channels of the optical signal even whileit is in service. The optical switching apparatus of the presentinvention requires only a simple configuration and procedure.

[0043] More preferably, the optical switch apparatus 100 of the presentinvention further optionally includes receiving circuits 101-1˜101-K,wavelength demultiplexers 102-1˜102-K, transponders or regenerators103-1˜103-K, wavelength multiplexers 108-1˜108-K and transmittingcircuits 109-1˜109-K depending on the condition such as the speed or thelevel of multiplexing of optical reception, which is transmitted andreceived through optical fiber, under which the optical switchingapparatus 100 of the present invention is used. In an alternativeembodiment of the optical switching apparatus of the present invention,the optical fibers 210-1˜210-K, 220-1˜220-K are directly connected tothe optical amplifiers 104-1˜104-N and the optical splitters or opticalcouplers 106-1 ˜106-N.

[0044] The aforementioned configuration in FIG. 2 is an embodiment of anOXC, which is one type of the optical switching apparatus according tothe present invention. In an OADM, which is another type of the opticalswitch apparatus, a partially separated or inserted optical signal inthe OADM, is directly inputted from the optical amplifiers 104-1˜104-Nand outputted to the optical splitters or optical couplers 106-1˜106-N,or is inputted to and outputted from the TDRs or the RGNs 103-1˜103-K.

[0045] Since the loss compensation for the optical switch 105 isrespectively made by the optical amplifiers and the monitor circuits onthe input and output sides of the optical switch, the loss compensationis not influenced by the peripheral optical fibers or the opticalsignals. Therefore, the optical switching apparatus of the presentinvention has few restrictions on the wavelengths of the optical signalsand the multiplexing methods.

[0046]FIG. 4 illustrates another embodiment of an optical switchingapparatus of the present invention. In the optical switching apparatus100′ of FIG. 4, the input circuits and output circuits of the OXCillustrated in FIG. 2 are simplified, and the configuration of thecontroller 110 is replaced by another controller 110′, which implementsthe control with firmware or software. Hereafter, the configuration ofthe controller 110′, which is different from the controller 110 in FIG.2, and the operation of controller 110′ will be explained. In FIG. 4,the components that are substantially identical to those in FIG. 2 arelabeled with the same number.

[0047] The controller 110′ includes an IO unit 130 and an operationmanagement unit 150. The IO unit 130 is connected to and communicateswith the operation management unit 150. The controller 110′ furtherincludes a CPU 131 which controls the controller 110′, a monitorselector 140, an amplifier controller 145, and a switch control unit 124for an optical switch 105 through a bus 136. Switch information memory132, which is an internal memory, stores the connection set-upinformation for the optical switch 105. An optical amplifier memory 133stores the control target value or the alarm information. Based on afirmware or software stored in a CPU memory (not illustrated), CPU 131controls each of the aforementioned units. Optionally, the switchinformation memory 132, the optical amplifier memory 133 and the CPUmemory reside on a single memory chip.

[0048] The monitor selector 140 includes analog-to-digital converters141 that convert a feedback signal from each one of the monitor circuits107-1˜107-N into digital data. The monitor selector 140 further includesa data storage device including a writing register 142 and a readingregister 143 that hold the digital data The monitor selector 140 furtherincludes a transfer control-unit 144 that controls the writing register142 and the reading register 143. Optionally, the transfer control unit144, the writing register 142 and the reading register 143 reside in theCPU 131 and/or the memory chip(s).

[0049] The amplifier controller 145 includes a comparator 146 thatcompares the digital data, which is the feedback from each one of themonitor circuit 107-1˜107-N with control target values in the opticalamplifier memory 133. A parameter-processing unit 147 selects aparticular one of the optical amplifiers 104-1˜104-N to be controlledbased on the above comparison result and generates control data. Adigital-to-analog converter 148 converts the control into analog signalsto control the particular one of the optical amplifiers 104-1˜104-N.Optionally, the comparator 146, the optical amplifier memory 133 and theparameter-processing unit 147 reside in the CPU 131 and/or the memorychip(s). Optionally, CPU 131 performs all of the processing steps of theamplifier controller 145 except for the-step of the A/D conversion.

[0050]FIG. 5 is an operational flow chart that describes the operationof the controller 110′ in the apparatus of FIG. 4.

[0051] The controller (110′) in the aforementioned configurationoperates as follows to compensate for optical signals. (1) Set-up ofoptical switch: Same as the steps S10 through S13 in FIG. 3. Holding theconnection set-up information in the switch information memory 132 inthe Step S13.

[0052] (2) Compensation for Optical Signal

[0053] a) The CPU 131 notifies the transfer control unit 144 for theparticular one of the monitor circuits 107-1˜107-N to be monitored andselect a particular area of the write register 142 to store the feedbacksignal from the monitor circuits 107-1˜107-N in Step S30. The transfercontrol unit 144 stores the digital data, that is the feedback signalfrom the selected one of the monitor circuits 107-1˜107-N and has beenconverted into digital data by the analog-to-digital converter 141 inthe particular area of the write register 144 in Step S31. The CPU 131repeats the steps S30 and S31 until all the paths have been set up inthe optical switch 105 using the predetermined rules such as cycles,supervisory orders, etc. and the connection information stored in theswitch information memory 132 Step S32.

[0054] b) Meanwhile, when one of the monitor circuits 107-1˜107-N isselected in Step S40 according to the predetermined rules such ascycles, supervisory orders, etc., the CPU 131 searches and selects aparticular one of the optical amplifiers 104-1˜104-N that is connectedto a particular input port based on the selected monitor circuit and theconnection set-up information in the switch information memory 132 inStep S41.

[0055] Through the transfer control unit 144, the digital data, that isthe feedback signal from the selected monitor circuit is transferredfrom the write register 142 to the readout register 143, and finally tothe comparator 146 in Step S42.

[0056] The comparator compares the digital data received in Step S42 andthe control target value obtained from the optical amplifier memory 133and generates a result in Step S43. The parameter-processing unit 147prepares the parameters based on the comparison result in Step S43 tocontrol the particularly selected one of the optical amplifiers104-1˜104-N, and controls the selected one of the optical amplifiers104-1˜104-N through the digital-to-analog converter 148 in Step S44 tocompensate for the loss and the differential loss among the differentchannels of the optical signals at the optical switch output port.

[0057] The above compensation steps S40 S44 are repeated until all theoptical paths in the optical switch 105 have been set up Step S45.Through its optical amplifiers 104-1˜104-N, the optical switchingapparatus 100′ of the present invention properly compensates the lossand differential loss in optical signals after they pass throughdifferent channels in the optical switch 105 by monitoring the outputports of the optical switch 105. In addition, a high-speed andhigh-capacity optical switching apparatus with a simple configurationand the procedure and method of using this apparatus are describe asfollows: Since the CPU 131 selects the one of the monitor circuits107-1˜107-N based on the content of the firmware or the software, theloss and the differential loss among the channels of the optical signalsis easily and securely compensated even while the apparatus 100′ is inservice. Furthermore, when the optical connection configuration of theapparatus is changed, such a change is easily incorporated by modifyingthe firmware or the software in the optical switching apparatus 100′.

[0058]FIG. 6 is an explanatory diagram that illustrates an example ofthe improvement of optical signals made by the optical switchingapparatus of the present invention. When the optical switching apparatus100 or 100′ of the present invention is implemented in a communicationnetwork, the actual loss or differential loss among the channels of theoptical switch is monitored on the output side of the optical switch.The monitored result is fed back to the optical amplifier at the inputside of the optical switch, and the optical switch is operated with thepre-compensated optical signal. Therefore, the loss that actuallyoccurred at the connection set-up or the switching of the optical switchis compensated, and an optical signal-to-noise ratio is kept constantindependent of the loss.

[0059] According to the optical switching apparatus of the presentinvention and method for use thereof, even if a high capacity opticalswitch is used in the optical switching apparatus, the loss and thedifferential loss among the channels of the optical signal through theoptical switch is monitored on the output port side of the opticalswitch. Therefore, the optical signal is properly compensated with asimple configuration and procedure using the optical amplifier on theinput port side of the optical switch based on the monitored feedbacksignal.

We claim:
 1. An optical switching apparatus that receives opticalsignals from a plurality of input circuits and outputs the opticalsignals to one of a plurality of output circuits comprising: a pluralityof input signal adjusting units for adjusting amplitude of the opticalsignals after being received at the plurality of the input circuits; anoptical signal switching unit having a plurality of input ports and aplurality of output ports, the plurality of the input ports beingconnected to said input signal adjusting units and receiving the opticalsignals after being adjusted in said input signal adjusting units, saidoptical signal switching unit transferring the optical signal from saidinput ports to one of the plurality of said output ports; a plurality ofoutput signal monitoring units connected to said output ports formonitoring the optical signals at said output ports to generate feedbacksignals based on the optical signals at said output port; and acontrolling unit, connected to the plurality of said input signaladjusting units, said optical signal switching unit and said outputsignal monitoring units for selecting at least one of said input signaladjusting units and at least one of said output signal monitoring unitsbased on a predetermined configuration of the optical switching unit andfor controlling said selected one of said input signal control unitsbased on the feedback signals generated by the selected one of saidoutput signal monitoring units.
 2. The optical switching apparatus asclaimed in claim 1, wherein said output signal monitoring units monitoran amplitude of the optical signals outputted from said opticalswitching unit to generate the feedback signals.
 3. The opticalswitching apparatus as claimed in claim 1, wherein said output signalmonitoring units monitor differential loss among different channelsoutputted from said optical switching unit to generate the feedbacksignals.
 4. An optical switching apparatus that receives optical signalsfrom a plurality of input circuits and outputs the optical signals toone of a plurality of output circuits, comprising: a plurality ofoptical amplifiers for amplifying the optical signals; an optical switchhaving a plurality of input ports and a plurality of output ports, saidinput ports being connected to said optical amplifiers, said opticalswitch transferring the optical signals received from said opticalamplifiers to said output ports; a plurality of monitor circuits formonitoring the optical signals at each of said output ports of saidoptical switch and generating feedback signal for said output ports; anda controller connected to the plurality of said optical amplifiers, saidoptical switch and said monitor circuits for selecting a particular oneof said monitor circuits based on predetermined rules, and a particularone of said optical amplifiers based on the selected one of said monitorcircuits and configuration of said optical switch, said controllercontrolling said selected one of said optical amplifiers based on thefeedback signals from the selected one of said monitor circuits.
 5. Theoptical switching apparatus as claimed in claim 4, wherein said outputsignal monitoring units monitor an amplitude of the optical signalsoutputted from said optical switching unit to generate the feedbacksignals.
 6. The optical switching apparatus as claimed in claim 4,wherein said output signal monitoring units monitor differential lossamong different channels outputted from said optical switching unit togenerate the feedback signals
 7. An optical switching apparatus thatreceives optical signals from a plurality of input and outputs theoptical signals to one of a plurality of output circuits, comprising: anoptical switch having a plurality of input ports and output ports, aplurality of input signal adjusting units for adjusting state of theoptical signals after being received at the input circuits, a pluralityof output signal monitoring units for monitoring the state of theoptical signals outputted to the output circuits, and a control unit forcontrolling said optical switch, said input signal adjusting units andsaid output signal monitoring units; wherein said input signal adjustingunits are respectively connected to said input ports of said opticalswitch, and wherein said output signal monitoring units are respectivelyconnected to said output ports of said optical switch, and wherein saidcontrol unit selects one of said output signal monitoring units toobtain the state of the optical signals at said output port, whereinsaid control unit selects a particular one of said input signaladjusting units based on a predetermined configuration of said opticalswitch, and whereby the selected one of said input signal adjustingunits adjusts the state of the optical signals.
 8. The optical switchingapparatus as claimed in claim 7, wherein said output signal monitoringunits monitor an amplitude of the optical signals outputted from saidoptical switching unit to generate the feedback signals.
 9. The opticalswitching apparatus as claimed in claim 7, wherein said output signalmonitoring units monitor differential loss among different channelsoutputted from said optical switching unit to generate the feedbacksignals
 10. An optical switching apparatus that receives optical signalsfrom a plurality of input circuits and outputs the optical signals toone of a plurality of output circuits, comprising: an optical switchhaving a plurality of input ports and output ports, a plurality ofoptical amplifiers for amplifying the optical signals received by theinput circuits, a plurality of monitor circuits for monitoring state ofthe optical signals to be outputted to the output circuits, and acontroller for controlling said optical switch, said optical amplifiersand said monitor circuits, wherein said optical amplifiers arerespectively connected to a corresponding one of said input ports ofsaid optical switch, and the monitor circuits are respectively connectedto a corresponding one of said output ports of said optical switch; andwherein said controller selects one of said monitor circuits to obtainthe state of the optical signals at said output ports, wherein saidcontroller selects a particular one of said optical amplifiers based ona predetermined configuration of said optical switch to amplify theoptical signals before the optical signals reach said input ports. 11.The optical switching apparatus as claimed in claim 10, wherein saidoutput signal monitoring units monitor an amplitude of the opticalsignals outputted from said optical switching unit to generate thefeedback signals.
 12. The optical switching apparatus as claimed inclaim 10, wherein said output signal monitoring units monitordifferential loss among different channels outputted from said opticalswitching unit to generate the feedback signals